As the demand on the device integration of an integrated circuit is gradually increased, the feature size of a semiconductor device (e.g. a field-effect transistor) becomes smaller and smaller, and the thickness of a gate oxide layer of the field effect transistor is reduced. For maintaining the dielectric performance and reducing current leakage, the gate oxide layer of the semiconductor device is usually made of a high-k material.
Moreover, since the doping capacity of the conventional poly-silicon gate electrode is limited, the efficacy of using the doped poly-silicon gate electrode to improve the threshold voltage is usually insufficient. Nowadays, for solving the problems resulting from reduction of the device feature size, the poly-silicon gate electrode is gradually replaced by a metal gate electrode.
However, this approach still has some drawbacks and problems. As to the process for forming a field-effect transistor with a metal gate electrode, it is necessary to remove a dummy poly-silicon gate electrode prior to forming a metal gate electrode to take the place of the dummy poly-silicon gate electrode. However, the process for removing the dummy poly-silicon gate electrode could damage a barrier layer overlaying on a gate dielectric layer of the field-effect transistor. Thus, current leakage of the field-effect transistor may be increased and punch through effect may occur due to metal atom penetrating through the damaged burrier layer and diffusing into the gate dielectric layer during the forming of the metal gate electrode.
Therefore, there is a need of providing an improved field-effect transistor and the fabricating method thereof to obviate the drawbacks encountered from the prior art.